Polyoma middle T antigen has been a workhorse for the study of signaling pathways critical to cancer. MT is the oncogene of polyoma virus, but is itself devoid of biochemical activity. It transforms by stealing host cell components. More importantly, the virus has been quite judicious in its choices of host pathways to attack, and consequently the study of MT binding proteins has proved fruitful. Notably PI3 Kinase, perhaps the leading candidate for tumor therapy at this time, was actually introduced to the world via the work of the Cantley, Schaffhausen and Roberts labs on MT. Notably what we call PI3 Kinase is actually composed of a small family of closely related lipid kinases. My colleagues and I have continued our studies of MT and PI3K, and have recently used conditional knockouts of the key catalytic subunits of PI3K, p110? and p110?, to discover a previously unknown subtlety in the roles of these enzymes in signaling and cancer. Both the p110? and p110? isoforms appear to play distinct roles in oncogenic transformation, and, interestingly, isoform functionality varies according to tumor type. The roles of the 2 isoforms in insulin signaling are also quite distinct suggesting that inhibiting individual isoforms could have fewer side-effects than the pan inhibitors now entering the clinic. Thus we are excited that the differences in the roles of the isoforms may be exploited to make safer second-generation drugs for PI3Ks. In the next grant period we propose to exploit the known strengths of MT to study the roles of PI3K isoforms in much greater detail. We want to use the well known ability of MT to transform cells of multiple tissue types to study the isoform requirements for MT mediated transformation. This will allow us to decipher whether isoform dependence on tumor initiation is determined by the tissue type or the nature of the oncogenic lesion(s). We plan to examine the isoform dependence of MT tumors in the breast and prostate. We will also follow up on preliminary data, which suggests that isoform specific PI3K inhibitors might be used as chemopreventatives to ameliorate familial cancers arising from PI3K pathway activation. Finally we will examine mechanisms by which cells become resistant to PI3K inhibition.